1. Field of the Invention
The present invention relates to a light isolator of the waveguide type which is suitably used in a light communication system or the like.
2. Related Background Art
In recent years, the necessity for light communication increases more and more with a rapid increase in information amount. The light communication system includes middle and long distance transmission systems using a laser beam of a band within a range from 1.3 to 1.5 .mu.m and a short distance transmission system such as an LAN (local area network) or the like using a laser beam of a band of 0.8 .mu.m. In any of the above cases, the light emitted from a semiconductor laser is converted onto an edge surface of an optical fiber and is waveguided in the fiber. At this time, the lights reflected by the fiber edge surface or the edge surfaces of other optical parts are returned to an active layer of the semiconductor laser. Thus, the oscillation of the semiconductor laser becomes unstable and a power fluctuation and a wavelength fluctuation occur. Particularly, a large influence is exerted on a distributed Bragg reflection (DBR) laser in a manner such that the single mode is changed to the multimode or the like. On the other hand, in the coherent light communication which is highlighted as a future communication system, information is expressed by merely changing the phase without executing the intensity modulation of the light. Thus, in particular, the coherent light communication is largely influenced by the returned lights.
To eliminate the returned lights, a light isolator is inevitable. At present, as a material of light isolator, a YIG (yttrium-iron-garnet) monocrystal or a paramagnetic glass in which rare earth ions are added is used. FIG. 1 shows a structure of a light isolator using a magnetooptical monocrystal which has been put into practical use at present.
Reference numeral 61 denotes a YIG monocrystal. A length l of crystal 61 is set to that a polarizing surface rotates by an angle of 45.degree. when a laser beam 65 of the linear polarization is transmitted in the crystal 61. That is, assuming that a Faraday rotation angle per unit length is set to .theta..sub.F, l is given by ##EQU1## Reference numerals 63 and 64 denote polarizing plates and their major axes are set so as to form an angle difference of 45.degree.. Reference numeral 62 denotes a hollow cylindrical permanent magnet. The magnetization of the magnet 62 is saturated by applying a bias magnetic field to the YIG monocrystal 61.
FIG. 2 shows an example of a light isolator of a conventional waveguide structure to realize a small size and low costs.
A laser beam 75 is waveguided in a magneto-optical monocrystalline thin film 71 which has grown on a monocrystalline substrate 72. At this time, the polarizing surface of the laser beam 75 is rotated by an angle of 45.degree. by the Faraday effect. In a manner similar to the light isolator shown in FIG. 1, there is an angle difference of 45.degree. between the major axes of polarizing plates 73 and 74.
The above waveguide type light isolator has an advantage such that it can be constructed in a small size and a light weight and interest has been shown in substituting it for a bulk type light isolator. On the other hand, the waveguide type light isolator is being developed as a key device when forming an OEIC (photoelectron integrated circuit).
However, the above waveguide type light isolator has the following drawbacks.
(I) According to the waveguide type light isolator which has been being developed at present, a Bi added YIG thin film is grown onto a GGG (gadolinium-gallium-garnet) monocrystalline substrate by an LPE (liquid phase epitaxy) method or a sputtering method. On the other hand, a compound semiconductor such as GaAs, InP, or the like is used as a subtract of the OEIC. It is difficult to epitaxially grow a YIG film as an oxide onto the substrate of GaAs or the like from a viewpoint of the differences of a lattice constant and a thermal expansion coefficient. Therefore, there is a large obstacle when such a method is applied to an integrated device.
(II) To develop the waveguide type light isolator, it is necessary to realize the phase matching between the TE wave and the TM wave.
FIGS. 3A to 3C show a method of realizing the phase matching. When a laser beam is waveguided in the film as shown in FIG. 3A, in the case of a certain limited film thickness h.sub.0, a refractive index to the TE wave is larger than that to the TM wave because of the shape birefringence as shown in FIG. 3B. A propagation constant difference .DELTA..beta. to the TE wave and TM wave is given by ##EQU2## where, .lambda.: wavelength of laser beam
n.sub.TE : refractive index to the TE wave PA1 n.sub.TM : refractive index to the TM wave
When the laser beam was waveguided by only the distance l, a mode conversion efficiency R from the TE wave to the TM wave is ##EQU3## where, .theta..sub.F : Faraday rotation angle per unit length.
As will be understood from the above equations, to improve the mode conversion efficiency, it is necessary to satisfy the condition of .DELTA..beta.=0, that is, n.sub.TE =n.sub.TM. Therefore, as shown in FIG. 3C, there has been continued an effort to realize n.sub.TE =n.sub.TM at a certain film thickness by giving the anisotropy in the direction perpendicular to the film surface by some means. Practically speaking, for instance, in the case of a garnet film, there has been executed a trial such as to provide a distortion inducing birefringence using a lattice constant difference between the substrate and the film or a growth inducing birefringence by controlling a temperature or compositions upon growing. However, to realize the phase matching by using those birefringences, it is necessary to strictly control the film forming conditions, so that the realization of such a method is not practical.
(III) Since the absorbing property of magnetic garnet for the light of 0.8 .mu.m is large, only the light isolator for the light of 1.3 to 1.5 .mu.m has been put into practical use at present. In the case of the garnet film, there is a limitation when a transmission factor for the light of 0.8 .mu.m is raised, so that it is demanded to develop a new material.
As described above, the waveguide type light isolator using a magnetic garnet film has a problem such that the integration with the laser is difficult. On the other hand, there is also a problem such that refractive indices to the TE wave and TM wave differ due to the shape birefringence, so that it is difficult to execute the phase matching. Further, since the absorption is large in the band of 0.8 .mu.m, there is a problem such that the magnetic garnet film cannot be used as a light isolator in such a band.